1. Field of the Invention
The present invention relates to tools for the formation of boreholes for cast-in-place piles in macroporous compressible soils.
2. Prior Art
The prior application by V. Feklin et al. discloses a tool for the formation of boreholes in macroporous compressible soils, which comprises a body adapted for connection to a drill rod, the body having a cylindrical calibrating portion, coaxial portions, confined by surfaces for the compaction of the soil, with the radii decreasing stepwise from the calibrating portion towards an end piece, and transition portions, defined by surfaces for a compaction of the soil, which form a smooth transition from the surface of a coaxial portion with a larger radius to the surface of an adjacent portion with a smaller radius. The surface for the compaction of the soil of each coaxial portion is a cylindrical surface described by a generatrix of a predetermined length, defined by two cylindrical helical lines having the same helix angle and serving as guides for the generatrix. The soil-compacting surface of each coaxial portion is thus a surface having the configuration of a ribbon wound from the calibrating portion in the adjacent coaxial portion of one radius and so forth in each coaxial portion up to the end piece, the "ribbons" of each coaxial portion being connected by the "ribbons" of the transition portions. In short, although the tool without the calibrating part is cone-shaped, the generatrix of the soil-compacting surface of each coaxial portion and of each transition portion is all the time parallel to the axis of the tool.
Such a construction of the tool makes it possible to form holes with compacted walls, which upgrades the quality characteristics of a hole and prompts increasing the bearing capacity of the cast-inplace piles made in such holes.
However, when such a tool is used to make holes for short cast-in-place piles, the calculated bearing capacity of the piles turns out to be lowered due to that the length of the cone-shaped part of the tool for the formation of short or not very deep holes amounts to 30 . . . 50% and more of their overall length, which reduces the total area of the side surface of cast-in-place piles made in such holes, since the area of the surface of a cone-shaped part with a base of a given diameter is less than the area of the surface of a cylindrical part of the same diameter and the same length or height.
The invention has as its aim to provide an improved tool for the formation of holes in macroporous compressible soils, which, owing to its novel configuration, promotes increasing the bearing capacity of piles, especially of short ones.